1. Field of the Invention
The present invention relates to an improved method for manufacturing semiconductor devices, and more particularly, to an improved atomic layer deposition (ALD) method for forming thin, high-quality HfO2 layers suitable for use as a dielectric material in such semiconductor devices.
2. Description of the Related Art
As the scale of semiconductor devices has decreased, the need for ultra-thin layers has gradually increased. However, the formation of ever thinner layers necessitates other process adjustments such as lower thermal budgets and the use of new materials. In addition to the thermal budget concerns, as the size of contact holes and other structural elements are decreased, problems associated with step coverage and loading effects tend to increase. Atomic layer deposition (ALD) methods have been proposed as a means for overcoming various problems resulting from the increased integration of semiconductor devices.
The basic ALD technique enables a material layer to be grown to a desired thickness by repeatedly forming very thin (i.e., atomic) layers of a desired material using two types of reactants sequentially applied to a reaction chamber. ALD techniques obtain an AB material layer by reacting minute quantities of two reactants, AX(g) and BY(g), on the surface of a substrate. The thickness of the AB material layer is increased by repeating the sequential supply and reaction of the AX(g) and BY(g) reactants generating XY(g) as a by-product. The deposition reaction can be generally represented by Formula I.AX(g)+BY(g)→AB(s)+XY(g)  I
More specifically, in an ALD process, a first reactant, AX(g), is supplied to a reactor chamber which a semiconductor substrate, such as a wafer, is provided. The first reactant AX(g) may be referred to as a “precursor” and is a compound obtained by combining an element A that will be used to form the desired AB material layer with another element or elements X. The first reactant AX(g), supplied to the reactor, may react with a surface of the substrate, or be chemically or physically adsorbed by the surface of the substrate. Here, since the absorption reaction can be regarded as the actual reaction, the first reactant layer of the chemically absorbed AX (or chemisorbed) is formed on the atomic-size level.
Next, the non-chemisorbed portion of first reactant AX(g) is removed from inside the reactor. This removal may be performed using vacuum exhausting or vacuum pumping. Alternatively, the vacuum exhaust process may comprise purging the reactor with an inert gas such as N2 or Ar. The purge cycle will remove substantially all of the physically-absorbed AX from the substrate and flush it, as well as any remaining non-absorbed first reactant AX(g) from the chamber. As a result, only the chemisorbed or reacted AX layer remains on the substrate.
A second reactant BY(g) is then supplied to the reactor. The second reactant B Y(g), which may also be referred to as a “precursor,” is a compound obtained by combining an element B necessary to form the desired AB material layer with an element or elements Y. A portion of the second reactant BY(g) reacts with the chemisorbed layer of AX according to Equation I, thereby forming a thin “atomic” AB(s) layer on the substrate and generating the by-product XY(g). The non-reacted portion of the second reactant BY(g) and the reaction by-product XY(g) are then removed from the reactor, typically by vacuum exhausting and/or purging with an inert gas.
The resulting AB(s) layer is formed roughly on the atomic-size level and is, therefore, not more than a few molecules thick. Accordingly, to form the AB material layer having the desired thickness, the cycle of supply, exhaustion, and purging of the AX(g) and BY(g) reactants is typically repeated several times.
Of particular interest in the quest for ever thinner and high-quality are the capacitive dielectric layers that are conventionally employed as gate dielectric layers within field effect transistors (FETs) and capacitor plate separation dielectric layers used in various types of microelectronic fabrications, including, for example, the production of highly integrated semiconductor devices. The capacitance of a structure may be determined using Formula II:C=εin·A/Tin  IIwhere C is the capacitance, εin is the dielectric constant of the dielectric material, A is the surface area of the capacitor and Tin is the thickness of the dielectric material. As reflected in this formula, as the area available for use as a capacitor is decreased in more highly integrated devices, the dielectric constant must increase and/or the thickness of the dielectric layer must decrease accordingly to maintain a similar level of capacitance.
While thinner dielectric layers are generally desirable in the search for improved performance of the capacitive structures used in semiconductor devices, the need for integrity, uniformity and dielectric strength in the dielectric layers can present significant technical barriers to the successful production of such materials. Traditional dielectric materials have included silicon oxides, silicon nitrides, silicon oxynitrides and composites or stacked structures of two or more such materials such as oxide-nitride-oxide (ONO) dielectrics.
These traditional dielectric materials, however, have a relatively low dielectric constant, typically between about 4 and 8, that limits the capacitance that can be obtained with layers being thick enough to maintain sufficient integrity. A number of alternative materials including a range of metal oxides, sometimes called high-k materials, typically having dielectric constants greater than 10, have been investigated and/or utilized to provide improved capacitive performance while allowing the use of thicker material layers. Silicon oxide is commonly used as a standard for comparing the performance of other dielectric materials whereby the thickness of an equivalent silicon oxide layer, i.e., the Toxeq, is calculated according to the Formula III:Toxeq=εox/εinTin  IIIin which εox is the dielectric constant of silicon dioxide, εin is the dielectric constant of the alternative dielectric(s) being used to form the capacitor and Tin is the thickness of the dielectric constant material.
While these alternative high-k dielectric materials such as metal oxides and metal-silicon oxides can be useful as high-performance dielectric materials, it has proven challenging to obtain such layers having sufficient purity and integrity using traditional chemical vapor deposition (CVD) methods employing organometallic precursors, i.e., compounds that contain both metal and carbon, as the source materials for the metal portion of the desired metal oxide because of the likelihood of contamination in the resulting material layers. In particular, capacitors incorporating such dielectric materials tend to suffer from carbon contamination resulting from residues of the organic portion of the organometallic precursor molecule that may degrade the leakage current characteristics of the resulting device.
One prior art solution to the problem of carbon contamination in metal oxide films is to follow the formation of the metal oxide film with an anneal process, generally in combination with a supplemental oxygen source, to “burn-off” the carbon contamination. Such a technique is disclosed in U.S. Pat. No. 6,395,650, the contents of which are incorporated herein by reference, in its entirety, in which the metal oxide film is irradiated with an ultraviolet radiation source such as an ultraviolet laser, an ultraviolet lamp or an ultraviolet plasma radiation source. Although the use of such methods may reduce the level of carbon contamination within the film, these methods also tend to increase the degree of crystallization within the metal oxide film, thereby creating potential alternative leakage paths along the grain boundaries.